In recent years, portable electronic devices such as smartphones and tablets have spread rapidly. Also with growing interest in the environment, hybrid cars and electric cars have attracted attention, thereby increasing the importance of secondary batteries. Examples of the secondary batteries include a nickel-metal hydride battery, a lead-acid battery, and a lithium-ion secondary battery. In particular, lithium-ion secondary batteries have been actively developed because the capacity thereof can be increased and the size thereof can be reduced.
A basic secondary battery includes a positive electrode, a negative electrode, and an electrolyte (an electrolyte solution or a solid electrolyte) provided therebetween. Typically, the positive electrode and the negative electrode each include a current collector and an active material layer provided over the current collector. In the case of a lithium-ion secondary battery, a material capable of receiving and releasing lithium ions is used as an active material for a positive electrode and a negative electrode.
As examples of positive electrode active materials of a lithium-ion secondary battery, phosphate compounds disclosed in Patent Document 1, such as lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), lithium cobalt phosphate (LiCoPO4), and lithium nickel phosphate (LiNiPO4), each having an olivine structure and containing lithium (Li) and iron (Fe), manganese (Mn), cobalt (Co), or nickel (Ni) are known.